325i N53 rotten egg smell and black smoke

Hi folks,

So my brother decided to have a drive of my e92 325i with N53 engine with 120k on the clock full BMW service history, me following him in his VW Scirocco 2.0 TSI (that has no relevance at all, but just in case you were interested). So he decides to overtake on a stretch of dual carriageway, obviously flooring it (which took off nicely, as the car is driving completely fine) But I notice a puff of dark smoke out the back of the exhaust as he starts to pull away on WOT, followed by a god awful stench of rotten eggs engulfing me.

So i'm really no expert, but I was always under the impression this smell could be due to the Catalytic converters coming to the end of their life span. So I guess i'm just asking for a bit of advice on:

1 - Could this smell be due to the cats?
2 - If so could the fact that the cats are on their way out lead to the puff of dark smoke?
3 - Any idea on pricing for getting these parts changed?
4 - I'm assuming the car has more than 1 cat, and if this is the case do they all need replacing or is there a way to diagnose if one is particular is defective?
5 - I may be completely wrong and one of you guys knows better???

Apologies in advance if I sound a little clueless, as I say i'm learning more about our cars every day thanks to this brilliant forum, and any help or advice would be greatly appreciated.

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325i N53 rotten egg smell and black smoke